Carbide coatings on carbon substrates are typically prepared by chemical vapor deposition (CVD). Generally, halide vapors of the carbide forming element are thermally decomposed at the substrate surface and reacted with a gas mixture. The gas mixture typically contains a volatile compound of the carbide precursor, such as methane, which acts as the source of carbon for the carbide layer, hydrogen, and an inert carrier gas such as argon. CVD yields a complete layer of uniform thickness of carbide over the carbon, however, there is minimal binding between the carbide coating and the carbon substrate. Consequently, differences in coefficients of thermal expansion of the carbide and carbon may result in the carbide coating peeling away from the substrate when the coated substrate is repeatedly cycled between high and low temperatures.
Furthermore, chemical vapor deposition of carbide coatings on carbon substrates must be performed using expensive and elaborate equipment such as ovens or induction furnaces operated at reduced pressures.
Other methods are known for reacting carbide forming elements in solution with porous graphite materials so as to fill up the porous graphite structures with carbides to create an impervious carbide structure. Carbide-treated graphite cuvettes for use as spectroscopic graphite electrodes to electrothermally vaporize samples in atomic absorption spectrophotometry have been prepared by impregnating porous carbon with metal chlorides.
As described by Almeida and Seitz in Applied Spectroscopy, 40(1), 4-8, 1986, porous graphite was impregnated with metal chloride by immersing the porous graphite in liquid metal chlorides under vacuum. The metal chlorides were hydrolyzed by soaking the impregnated porous graphite in water. The hydrolyzed precursor was then dehydrated in an electrothermal atomizer to dry the metal hydroxide and then heated to form the carbide.
Such a process required the time consuming steps of hydrolyzing by soaking in water (for 24 hours) and heating (for 2 hours) to dehydrate the metal hydroxide. Further, such a process resulted in extensive oxide formation. Most notably, only porous substrates could be impregnated by such a process.